Method for impregnating cellulosic material



Dec. 27, 1966 A. w. GESSNER METHOD FOR IMPREGNATING CELLULOSIC MATERIAL Filed March 28, 1963 INVENTOR ADOLF W. GESSNER BY 77a TEMPERATURE F ATTORNEY United States Patent 3,294,625 METHOD FOR IMPREGNATING CELLULOSIC MATERIAL Adolf W. Gessner, Moutclair, N.J., assignor to The Lummus Company, New York, N.Y., a corporation of Delaware Filed Mar. 28, 1963, Ser. No. 268,623 7 Claims. (Cl. 162-68) The present invention relates to an improved method for treating cellulosic material, and more particularly, the present invention relates to an improved method for the impregnation of cellulosic material, such as wood chips and the like with cooking liquor.

It has long been recognized that one of the major causes of non-uniform pulp is irregular penetration of cooking liquor into the cellulosic material prior to delignification. 'If the cellulosic material is soaked in delignifyin-g liquor at low temperature and atmospheric pressure, the time required Lfor thorough impregnation is excessive. On the other hand, if the cellulosic material is soaked at temperatures and pressures necessary for delignification, only the surface portions of the chips are impregnated prior to the start of delignification, and consequently the interior portion of the chips is not fully treated, which results in an inferior pulp product.

The present invention provides an improved method for impregnating cellulosic material whereby water vapor within the pores of the material is condensed, causing the impregnating liquor to be drawn into the pores of the material. Accordingly, wood chips or the like are introduced into a deaerating vessel to remove air from such material by the introduction of low pressure steam into the vessel. The chips are then passed into an impregnation vessel having a vapor space above the surface of the liquor. The temperature of the liquid in the impregnation vessel is slightly lower than the temperature of the steam introduced into the deaerating vessel, and the total pressure within the impregnation vessel is maintained slightly lower than the pressure in the deaerating vessel. The water vapor within the chips expands to the total pressure in the impregnation vessel as the chips enter and fall through the vapor space in the impregnation vessel. When the chips contact the slightly cooler liquor, they are cooled and the water vapor within the chips condenses, thereby forming a lower pressure in the pores of the chips which draws cooking liquor into the pores by the pressure difiFerence. The condensation of water vapor in the pores of the chips occurs at the dew point corresponding to the total pressure in the impregnation vessel. Since the liquor is cooler than the condensation temperature, and since the liquor contains dissolved chemicals, the saturation partial pressure of water vapor above the liquor is appreciably below the total pressure in the impregnation vessel. Air or other non-condensible gases are therefore introduced into the vapor space so that the total pressure in the impregnation vessel is above the equilibrium partial pressure of water vapor above the liquor. The chips, when thoroughly impregnated with liquor, have a specific gravity somewhat above that of the liquor, and thus sink beneath the surface of the liquor without mechanical aid.

One of the primary objects of the present invention is to provide an improved method of impregnating cellulosic material. with delignifying liquor whereby a more uniform pulp is attained.

Another object of the present invention is to provide an improved method of impregnating cellulosic material wherein water vapor in the pores of the cellulosic material is condensed, thereby effecting a lower pressure Within the pores which draws delignifying liquor into the pores of the material.

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Still another object of the present invention is to provide a method wherein the pores of cellulosic material or the like being impregnated are filled with a delignifying liquor rather than a non-delignifyin-g liquor, such as water, into which the chemicals of the delignifying liquor would have to penetrate by the slower process of diffusion.

It is another object 'of the present invention to impregnate a cell-uosic material with a delignifying liquor within an impregnation zone wherein the vapor space above the delignifying liquor is maintained at a pressure greater than the water vapor pressure above the delignifying liquor.

Other objects, features, and advantages of the present invention will become more apparent from the follow-' ing description when taken in conjunction with the drawing wherein,

FIGURE 1 is a diagrammatic view of an apparatus for carrying out the present invention, and

FIGURE 2 is a vapor pressure vs. temperature diagram illustrating the operating conditions in accordance with the present invention.

Referring now to the drawing, and more particularly to FIGURE 1, wood chips or like cellulosic material are passed from a chip supply tank 10 through a star valve 12 or other suitable metering device into a deaerating tank 14. Low pressure steam is introduced through a steam supply line 16, provided with a control valve 18 into the :bottom of the tank 14. A vent line 20 provided with a control valve 22 is in communication with an upper portion of tank 14. Valve 18, controlled by suitable means (not shown), permits steam to be introduced into the tank 14 to subject the chips to the steam pres sure. Valve 22, controlled by suitable means (not shown), permits the air in the tank 14 to be withdrawn when the chips are deaerated by replacing the air within the chips with water vapor.

Deaerated chips from tank 14 are withdrawn through -a star valve 24 or other suitable metering device. Should slide valves be used instead of the star valves 12 and 24, the chips are subjected to batchwise steaming in tank 14, by sequentially opening and closing the valves 18 and 22, by suitable control means (not shown), so that the chips in the tank are alternate-1y subjected to the steam pressure and then vented to efiFect deaeration and the replacement of air in the chips with water vapor.

The deaerated chips withdrawn from the tank 14 are passed into an impregnation vessel, generally designated by reference numeral 26. Cooking liquor for impregnating the chips is supplied to vessel 26 through line 28 controlled =by valve 30. The liquor level 32 in tank 26 is regulated by suitable liquid level control means (not shown) operatively connected to valve 30 to maintain a vapor space 34 in the top of vessel 26. The pressure in vessel 26 is maintained slightly below the pressure of the deaerating tank 14. The temperature of the cooking liquor in the vessel '26 is maintained slightly below the dew point of water vapor at the total pressure within the vessel 26. Since the water vapor pressure at the temperature of the cooking liquor within the vessel 26 is less than the total pressure, an inert gas, such as air, is introduced into the vessel 26 so that the total pressure is slightly below the pressure within the deaerating tank 26. Consequently, compressed air or a similar inert gaseous medium is introduced into the vapor space of vessel 26 through line 36 controlled by valve 38 which may be regulated by a suitable pressure or flow con troller 40.

Gases may be exhausted from the vapor space through line 42 controlled by valve 44 which is regulated by a pressure controller 46. Controllers 40 and 46 maintain the pressure in vapor space 34 at a pressure slightly lower than the pressure within the tank 14. As the chips fall through the vapor space, the water vapor in the pores of the chips expands thereby preventing air infiltration. The chips then contact the cooking liquor and are cooled causing the water vapor in'the pores .to condense creating a lower pressure within the chips which draws in the cooking liquor. T'he impregnated chips now having a specfic gravity somewhat higher than that of the liquor, sink beneath liquor surface 32, and are subsequently withdrawn as a suspension from vessel 26 through line 48 for subsequent digestion.

A specific operating example will now be described with reference to the vapor pressure curves of FIGURE 2 wherein the curve A is the partial pressure of water vapor above pure water, and the curve B is the partial pressure of Water vapor above the liquor. Generally, the chemicals in the liquor will cause a boiling point elevation of about 3.l F. to 3.5 F. FIGURE 2 is based on a 35 F. boiling point elevation.

Wood chips or like cellulosic material are introduced into the deaerating vessel 14 maintained at a pressure of 45 p.s.i.a. Saturated steam at 45 p.s.i.a. is introduced through line 16. The chips leave the deaerating vessel 14 with their pores essentially filled with saturated steam and some steam condensate at 274.4 F. and 45 p.s.i.a. These conditions are indicated by the point F on the curve A in FIGURE 2. The pressure in vapor space 34 is maintained at 40 p.s.i.a., by means of pressure controllers 40 and 46. The temperature of the cooking liquor in impregnation vessel 26 is maintained at 260 F. The partial pressure of water vapor above liquor at this temperature is about 33.4 p.s.i.a. These conditions are designated by point H on FIGURE 2. Since the partial pressure of water vapor is 33.4 p.s.i.a., air is introduced through line 36 to maintain the total pressure in the vapor space at 40 p.s.i.a. Thus, the partial pressure of the air within the vessel 26 is 6.6 p.s.i.a.

As the chips fall through the vapor space, the water vapor in the pores of the chips expands as a result of the lower pressure in the vapor space and causes an outflow of steam from the pores of the chips. Consequently, there will be no air infiltration into the chips. The condition of the steam within the chips after expansion to 40 p.s.i.a. is approximately represented by point G on FIG- URE 2. The dew point of the steam in the chips at 40 p.s.i.a. is 267.25 F. and is represented by point I on FIG- URE 2. Upon contacting the liquor, the chips are cooled from the conditions of point G toward the point I. Since the dew point of the Water vapor in the chips is above the temperature of the liquor, all of the water vapor must condense before the chips can attain any temperature below 267.25 F. As the chips approach point I, the water vapor within the pores condenses, thus contracting in volume and drawing the cooking liquor into the pore volume of the chips by the pressure difference.

It will be appreciated that the operating conditions may be varied so long as they are in keeping with the essential features described above including maintaining the total pressure in the vapor space of the vessel 26 below the pressure in the deaerating vessel 14, and to maintain the cooking liquor temperature slightly below the condensation temperature of Water vapor at such total pressure, making up the diiference between the vapor pressure above the cooking liquor and the total pressure by introducing an inert gas, such as air, into the impregnating vessel. It is thus apparent that the present invention is susceptible of many changes and modifications, and it is intended to encompass all such changes and modifications as fall within the scope of the appended claims.

I claim:

1. A process for impregnating wood chips or the like, comprising replacing air within the pores of the chips with water vapor in a deaerating zone,

introducing the deaerated chips into an impregnating zone having a body of cooking liquor at a temper- 4 ature of from about 240 F. to about 260 F. and a vapor space above said body of liquor, introducing an inert gas into said vapor space to maintain the pressure in said vapor space at about 2 to 5 about 6 p.s.i.a. above the vapor pressure of said liquor at the operating temperature,

maintaining the pressure in said vapor space at about 1 to about 5 p.s.i.a. below the pressure of said deaerating zone,

and contacting said chips with said liquor to condense the water vapor in said chips to effect a lower pressure within said pores, thereby drawing said liquor into said chips. 2. A process for impregnating wood chips or the like, comprising replacing air within the pores of the chips with water vapor in a deaerating zone,

introducing the deaerated chips into an impregnating zone having a body of cooking liquor at about 260 F. and a vapor space above said body of liquor,

introducing an inert gas into said vapor space to maintain the pressure in said vapor space about 6 p.s.i.a. above the vapor pressure of said liquor at the Operating temperature,

and contacting said chips with said liquor to condense the water vapor in said chips to eifect a lower pressure within said pores thereby drawing said liquor into said chips.

3. A process for impregnating cellulosic material comprising replacing air within the pores of the material With water vapor in a deaerating zone,

introducing the deaerated material into an impregnating zone having a body of liquor and vapor space above said body of liquor, said deaerated material first passing through said vapor space,

maintaining the pressure in said vapor space above the vapor pressure of said liquor, maintaining the pressure in said vapor space below the pressure of said deaerating zone, and contacting the material with said liquor to condense said water vapor in said material to effect a lower pressure Within said pores, thereby drawing said liquor into said material.

4. A process for impregnating cellulosic material comprising replacing air within the pores of the material with water vapor in a deaerating zone,

introducing the deaerated material into an impregnating zone having a body of liquor and a vapor space above said body of liquor, said deaerated material first passing through said vapor space,

maintaining the pressure in said vapor space slightly above the vapor pressure of said liquor and below the pressure in the deaerating zone by introducing a gas into said space, maintaining the temperature of said liquor slightly below the dew point of the water vapor in the material,

and contacting the material with said liquor to condense the water vapor in said material to efiect a lower pressure within said pores, thereby drawing said liquor into said material.

5. A process for impregnating cellulosic material comprising replacing air within the pores of the material with Water vapor in a deaerating zone,

introducing the deaerated material into an impregnating zone having a body of liquor and a vapor space above said body of liquor, said deaerated material first passing through said vapor space, I

maintaining the pressure in said vapor space slightly above the vapor pressure of said liquor,

and contacting the material with said liquor to condense said water vapor in said material to effect a lower pressure within said pores, thereby drawing said liquor into said material.

6. A process for impregnating cellulosic material com- 5 prising replacing air within the pores of the material with Water vapor in a deaerating zone,

introducing the deaerated material into an impregnating zone having a body of liquor and vapor space above said body of liquor, said deaerated material first passing through said vapor space, maintaining the pressure in said vapor space slightly above the vapor pressure of said liquor, and lower than the pressure of said deaerating zone,

maintaining the temperature of said liquor slightly below the dew point of the water vapor in said material,

and contacting the material with said liquor to condense said water vapor in said material to efiect a lower pressure within said pores, thereby drawing said liquor into said material.

7. A process for impregnating cellulosic material comprising replacing air within the pores of the material with water vapor in a deaerating zone,

introducing the deaerated material into an impregnating zone having a body of liquor and a vapor space above said body of liquor,

maintaining the pressure in said vapor space slightly above the vapor pressure of said liquor, maintaining the temperature of said liquor at about References Cited by the Examiner UNITED STATES PATENTS Titfany. Maste. Wells. Codwise. Rue. Olsen. Ross.

FOREIGN PATENTS Great Britain.

DONALL H. SYLVESTER, Primary Examiner.

MORRIS O. WOLK, Examiner.

H. R. CAINE, Assistant Examiner. 

1. A PROCESS FOR IMPREGNATING WOOD CHIPS OR THE LIKE, COMPRISING REPLACING AIR WITHIN THE PORES OF THE CHIPS WITH WATER VAPOR IN A DEAERATING ZONE, INTRODUCING THE DEAERATED CHIPS INTO AN IMPREGNATING ZONE HAVING A BODY OF COOKING LIQUOR AT A TEMPERATURE OF FROM ABOUT 240*F. TO ABOUT 260*F. AND A VAPOR SPACE ABOVE SAID BODY OF LIQUOR, INTRODUCING AN INERT GAS INTO SAID VAPOR SPACE TO MAINTAIN THE PRESSURE IN SAID VAPOR SPACE AT ABOVE 2 TO ABOUT 6 P.S.I.A. ABOVE THE VAPOR PRESSURE OF SAID LIQUOR AT THE OPERATING TEMPERATURE, MAINTAINING THE PRESSURE IN SAID VAPOR SPACE AT ABOUT 1 TO ABOUT 5 P.S.I.A. BELOW THE PRESSURE OF SAID DEAERATING ZONE, AND CONTACTING SAID CHIPS WITH SAID LIQUOR TO CONDENSE THE WATER VAPOR IN SAID CHIPS TO EFFECT A LOWER PRESSURE WITHIN SAID PORES, THEREBY DRAWING SAID LIQUOR INTO SAID CHIPS. 